The present invention relates to a laser diode package having a monitor diode for monitoring an optical power of a laser diode, and more particularly, to a laser diode package having an optical power monitoring function for shielding ambient light which can induce a misoperation of a monitor diode.
A conventional optical disc system, such as a compact disc player, projects laser light onto the recording surface of a disc and reads information recorded on the disc. In such an optical disc system, if an output of the laser light varies, an error may be generated when a signal is read from the disc. Thus, the optical disc system includes an automatic laser power control (ALPC) to stably control the output of the laser light.
A semiconductor laser diode used as a light source in a compact disc player has a poor temperature characteristic. As the temperature rises, the optical power decreases, while as the temperature falls, the optical power increases. The ALPC circuit in the conventional optical disc system uses a photodiode to monitor the optical power of such a laser diode, and controls a driving current of the laser diode according to the current flowing through the photodiode.
FIG. 1 shows the structure of a conventional laser diode package. The laser diode package 100 of FIG. 1 includes a laser diode 11, a monitor diode 12 for monitoring an optical power of the laser diode 11 and a signal detection diode 13 for detecting information recorded on a disc 17. The laser diode 11, the monitor diode 12 and the signal detection diode 13 are mounted on a lead frame base 10. Most of the light emitted from the laser diode 11 passes through a transparent plate 15 and then is incident to an objective lens 16. Part of the light emitted from the laser diode 11 is reflected from a surface 18 of the transparent plate 15 and then is incident to the monitor diode 12. The light incident to the objective lens 16 is focussed at a point on the disc 17 by the objective lens 16. The light reflected from the disc 17 enters into the laser diode package 100 via the objective lens 16.
The transparent plate 15 of the laser diode package 100 has a structure in which a holographic optical element 14 is installed on or near the surface facing the objective lens 16, and is fabricated by injection molding. The holographic optical element 14 of the transparent plate 15 diffracts the light incident from the objective lens 16 and the light diffracted by the holographic optical element 14 is incident to the signal detection diode 13.
The monitor diode 12 outputs a monitor current to an ALPC circuit (not shown). The monitor current is proportional to the quantity of the incident light. The well-known ALPC circuit adjusts the driving current of the laser diode 11 uniformly according to the monitor current output from the monitor diode 12.
When the disc 17 wobbles or ambient light "a" is incident to the monitor diode 12, the monitor diode 12 monitors the ambient light "a" as well as the quantity of the light incident from the laser diode 11 so that more than the actual quantity of light incident from the laser diode 11 is recognized, thus the improper monitor current is output to the ALPC.